The present invention relates to an elongating method that employs a mandrel mill for the manufacture of metal tubes, in particular seamless tubes, as well as an apparatus for implementing that method. The following description is directed to seamless steel tube as a typical example of "metal tube".
The steps for the production of a seamless steel tube of the prior art are first described below.
As shown in FIG. 1, facilities commonly employed in the art comprise a rotary hearth furnace A, a piercing mill (Mannesmann piercer) B, an elongator (mandrel mill) C, a reheating furnace D, and a reducing mill (stretch reducer) E.
A round steel billet 1 emerging from the heating furnace A is first pierced with the Mannesmann piercer B. The thus rolled hollow piece 2, which is rather short and thick-walled, is fed to the mandrel mill C, in which the hollow piece, with a mandrel bar 3 inserted, is continuously rolled between grooved rolls 4 to reduce its wall thickness whereas its length is elongated to produce a hollow shell 5.
Since the temperature of the hollow shell 5 drops during the rolling operation, the shell is reheated in the reheating furnace D before it is sent to the reducing mill (stretch reducer) E where its outside diameter is reduced to a predetermined final dimension with rolls 6.
The operation on the mandrel mill C at the elongating stage of this production process is further described below.
Mandrel mill C is a rolling mill on which the hollow piece 2 that has been pierced with the Mannesmann piercer B and which has the mandrel bar 3 inserted thereinto is subjected to an elongating action.
The mill usually consists of 6-8 stands that are each inclined at 45.degree. to the horizontal and which are staggered from each other by 90.degree. in phase; this "X" mill structure is common in the art. As the hollow piece 2 is passed through all stands in the mandrel mill C, its length is elongated by a factor of about 4 times at maximum.
The early type of mandrel mill was a "full floating" mandrel mill which, as mentioned above, was used in continuous rolling of a hollow piece 2 by means of grooved rolls 4, with mandrel bar 3 inserted into the hollow piece. In the period from 1977 to 1978, a "retained" (also known as "restrained") mandrel mill was developed and commercialized. This new type of mandrel mill which can achieve higher efficiency and quality was introduced at plants in many countries of the world to manufacture small and medium-diameter seamless steel tubes.
In the retained mandrel mill, mandrel bar retainer C-1 retains or restrains the mandrel bar 3 from its rear end until the end of rolling. According to the manner in which the mandrel bar 3 is handled after the end of rolling, the retained mandrel mill is classified as a semi-floating type in which the mandrel bar 3 is released simultaneously with the end of rolling or as a full-retracting type in which the mandrel bar 3 is pulled back simultaneous with the end of rolling. The semi-floating type is common in the manufacture of small-diameter seamless steel tubes whereas the full-retracting type is common in the manufacture of medium or large-diameter seamless steel tubes.
In the full-retracting type, extractor C-3 is connected to the delivery end of mandrel mill C so that while a rolling operation is underway in mandrel mill C-2, the hollow shell 5 is extracted, or pulled out of the mandrel mill C-2 with the extractor C-3. If the temperature of the tube material emerging from the delivery end of the mandrel mill C-2 is sufficiently high, the reheating furnace D is unnecessary.
Thus, in the retained mandrel mill, whether it is of a full retracting type or a semi-floating type, the mandrel bar is retained and/or restrained from its rear end during rolling. Hence, the elongated hollow shell has such a nature as to readily separate from the mandrel bar, and a closed roll pass that has a correspondingly increased degree of roundness can be adopted, which contributes to a marked improvement in the circumferential uniformity of the wall thickness of the tube.
In an early full-floating mandrel mill, the direction of the frictional force acting on the inner surface of the tube varies constantly during the transient state, i.e., when the leading end of the tube is gripped by rolls or when the trailing end of the tube leaves the mill. As a result, a compressive force is said to act between stands to cause an undesired phenomenon called "stomach formation". This "stomach formation" problem has successfully been solved by the new retained mandrel mill since it enables a frictional force to keep on the inside surface of the shell at all times in a constant direction.
Thus, the use of the retained mandrel mill has been a solution to the "stomach formation" problem. However, all types of mandrel mills that are used today have a major problem that it is necessary to keep a huge number of mandrel bars in stock.
More specifically, the common practice with the mandrel mill, whether of a full-floating type, a semi-floating type, or a full-retracting type, is to adjust the wall thickness of the tube by changing the diameter of the mandrel bar while maintaining the roll opening, or the gap between the top and bottom grooved rolls at a constant level. Since the roll opening cannot be varied to adjust the wall thickness as in the case of rolling plates or strips, a huge number of mandrel bars must be made available at the shop in order to roll hollow shells of varying outside diameters over a wide range of wall thicknesses (including heavy and light-wall tubes).
The reason why wall thickness changes cannot be made with a mandrel mill by adjusting the roll opening is as follows.
The shape of a mandrel bar is a true circle whereas the shape of a roll pass is elliptic. Hence, the space between the roll pass and the mandrel bar will naturally be nonuniform in the circumferential direction. As a result, the wall thickness will increase in a position that is approximately 30.degree.-45.degree. inclined with respect to the oval direction of the roll pass, i.e., in a position at the point of wall thickness separation where the inner surface of the shell leaves the mandrel bar, so that the circumferential width of the roll pass will increase at the groove side and decrease at the flange side, thereby increasing the chance of projections forming on the inside surface of the tube at the flange side. A typical example of this phenomenon is shown in FIG. 2. Obviously, the tube wall 10 is provided with four inner projections 12 that are symmetric with respect to both the horizontal and the oval axis.
This problem generally called "quarter-projections" is inherent in mandrel mills and can be eliminated by a suitable pass design. However, if one attempts to alter the wall thickness by reducing the roll opening while using mandrel bars of the same diameter, the projections on the inner surface of the shell will appear further until the geometry of the tube is greatly deteriorated.
The common practice adopted today to change the wall thickness of a hollow shell with a mandrel mill, therefore, is to alter the diameter of the mandrel bar while maintaining the roll opening constant. This necessitates the use of a huge number of mandrel bars, and as many as 5000 mandrel bars are provided at a shop for producing small-diameter seamless steel tubes up to sizes of about 7 inches. For rolling seamless steel tubes ranging from small to medium or large size (around 5-16 inches), 10,000 mandrel bars must be provided. Hence, a very large automated warehouse becomes necessary just for keeping mandrel bars, and this increases not only the initial investment but also the running costs for the repair and maintenance of mandrel bars.